There have been recent breakthroughs in preparing ferroelectric devices comprising ferroelectric potassium nitrate. These devices are particularly useful as computer memory cells and preferably employ Phase III potassium nitrate in the ferroelectric phase which, in thin film form, is stable at ordinary room temperature and pressure. U.S. Pat. Nos. 3,728,694 and 3,939,292 disclose the preparation of such memory devices in detail and their teachings are incorporated herein by reference.
Various ferroelectric materials have been studied for their information storage capability. Although many materials exhibit the ferroelectric phenomenon, the predominant materials previously studied for memory application are barium titanate, potassium dihydroxide phosphate, tri-glycerine sulfate, and Phase III potassium nitrate. Phase III potassium nitrate exhibits a well defined critical switching threshold. Three conditions which must be fulfilled for a crystalline material to exhibit ferroectricity are as follows:
1. It must have a phase transition from a polar to a non-polar structure, or at least must tend, with rising temperature, toward such a transition.
2. The polar phase must have a spontaneous polarization, that is, the unit cell must actually have a dipole moment, not only belong to a space group which is capable of such a moment.
3. The direction of the spontaneous polarization must be reversible by the applied electric field. This third condition is the most important.
The ferroelectric portion of this application and the parent application is in the form of a capacitor memory cell. Geometrically, the memory cell is a capacitor with upper and lower metal electrodes sandwiching the ferroelectric material as the dielectric. As taught in U.S. Pat. Nos. 3,728,694, the ferroelectric material should be less than 110 microns and preferably have a thickness within the range of from 100 Angstrom units to 1,000 Angstrom units. When the ferroelectric material is Phase III potassium nitrate, a thickness of less than 1 micron is preferred in order to achieve fast switching times. Fabricating multilayered devices such as these including metal layers are usually accomplished utilizing high vacuum deposition techniques.
The fabrication of semiconductor integrated circuits are well known and conventional. As used herein, the term "semi-conductor integrated circuit" is intended to include, inter alia, MOS and bipolar designs. These devices have also, in the past, been used in conjunction with various memory devices. Never before, however, have semiconductor integrated circuits been fabricated with thin film ferroelectric memory devices in a monolithic structure. Furthermore, semiconductor integrated circuits have never been placed within a monolithic structure including a thin film ferroelectric memory device, preferably including Phase III potassium nitrate as the dielectric.
The monolithic semiconductor integrated circuit and ferroelectric memory device of the present application is capable of at least 10.sup.10 read/write cycles of operation without failure. If, however, a more long-lasting memory device is sought, certain modifications are necessary which form another aspect of the present invention. More specifically, it was found that beyond 10.sup.10 read/write cycles, the metal electrodes which sandwich the potassium nitrate layer eventually oxidize in response to a chemical reaction between the KNO.sub.3 and the electrodes. This oxidation reaction is accelerated in the presence of an electric field and current flow through the ferroelectric layer.
It was also found that occasional failures occur due primarily to cracks in the KNO.sub.3 layer. The electrode materials have a tendency to migrate into these cracks or imperfections and short out the device when electrode materials on one side of the ferroelectric layer contact electrode materials on the opposite side of the ferroelectric layer.
The use of semiconductor integrated circuits fabricated with thin film ferroelectric memory devices in a monolithic structure requires the use of the unique processing steps which forms one aspect of the present invention. It was found that the ferroelectric memory layer, particularly Phase III potassium nitrate, is particularly sensitive to liquids such as water. This fact has lead to the need for development of a unique method of fabricating the semiconductor integrated circuit-ferroelectric layer in order to produce a structure which is of practical utility.